1. Field of the Invention
The present invention relates to a hill hold control apparatus for a vehicle and more particularly to a hill hold control apparatus capable of locking wheels when a selector lever of an automatic transmission is positioned at xe2x80x9cforward or reverse drivexe2x80x9d range.
2. Description of Background Arts
Japanese Patent Application Laid-open No. Toku-Kai 2000-65199 discloses an automatic transmission for an automobile comprising an input shaft having a plurality of drive gears, an output shaft having a plurality of driven gears paired with respective drive gears and a bypass clutch for engaging the input shaft with the output shaft. This type of automatic transmission has an advantage in that the rotation speed of the output shaft can be synchronized with the engine speed and shift shocks at up-shifting can be reduced by preventing so-called xe2x80x9ctorque dropxe2x80x9d by transmitting torque through the bypass clutch in high speed shift stages. However, this type automatic transmission, in case where a friction clutch is used as a start clutch for engaging a crank shaft with the input shaft, has a problem that when the vehicle attempts to start forward on a grade, the vehicle reverses, while a driver shifts his or her foot from a brake pedal to an accelerator pedal and the friction clutch which has been released at stopping is engaged again.
On the other hand, automatic transmissions of normal torque converter type having planetary gears generate creep torque. If a vehicle stops on a steep grade, creep torque can not hold the vehicle at a standstill and the vehicle reverses. Japanese Patent Application Laid-open No. Toku-Kai 2000-127928 discloses a hill hold control apparatus capable of locking wheels by a brake system when the select lever of an automatic transmission is selected at a forward drive position and specified conditions are satisfied. However, this hill hold apparatus needs to incorporate an accumulator and an electromagnetic operative opening and closing valve in the apparatus, this leading to an increase of the number of components and manufacturing cost.
It is an object of the present invention to provide a hill hold control apparatus for a vehicle capable of performing a hill hold control easily by utilizing a bypass clutch provided between an input shaft and an output shaft of an automatic transmission.
To achieve the object, a hill hold control apparatus comprises a grade judging means for judging that the vehicle travels on a grade, a shift gear train detecting means for detecting a shift gear train of the automatic transmission, a foot brake operation detecting means for detecting an operation of a foot brake, a first control means for partially engaging the bypass clutch when it is judged that the vehicle is in standstill and the shift gear train is a start gear train and the foot brake is operative based on respective signals of the grade, the shift gear train and the operation of the foot brake and a second control means for engaging the bypass clutch when it is judged that the vehicle is in standstill and the shift gear train is a start gear train and the foot brake is inoperative based on respective signals of the grade, the shift gear train and the operation of the foot brake.